This invention relates to an improved process for producing an amended sodium carbonate product. More particularly, this invention relates to the manufacture of a sodium carbonate product, solids and/or liquor, that has an enhanced reactivity with carbon dioxide for the production of sodium bicarbonate free of the use during manufacture of additives that includes animal derivatives.
Trona ore is mined and calcined for use in the manufacture of sodium carbonate which in turn can be used to make sodium bicarbonate (NaHCO3), a valuable product. The naturally occurring trona ore material generally has the formula Na3H(CO3)2.2H2O and is characterized as a hydroxyacid sodium carbonate. Trona is found in, or contiguous to, oil shale, and thus, contains large amounts of organics, which it is desirable to remove from the sodium carbonate product. Unfortunately, insoluble organic and inorganic materials are contiguous in the trona ore, and are not easily separated. These impurities impact the characteristics of the final soda ash produced.
The processes used to remove impurities and to produce commercial soda ash from crude trona ore include various steps of calcination, dissolution of the converted soda ash to concentrated liquor, solids/liquids separation steps, filtration and/or purification, evaporation/crystallization, and drying the monohydrate formed to anhydrous soda ash for industrial use.
In accordance with the present commercial process, the crushed and calcined trona ore is treated with water to dissolve the soluble sodium carbonate product. The resultant liquid solution, or liquor, is clarified, decanted and then filtered to remove the solids. Treatment of the solution with activated carbon may follow to remove some portion of the organic materials. However, treatment with activated carbon is expensive. In addition to the high costs of the activated carbon itself, there are several auxiliary processing costs; the carbon must be filtered out after the carbon is sufficiently inactivated, requiring additional manpower, testing and filtering equipment, and the spent carbon must be disposed of, which is also expensive.
After the carbon treatment step, when used, the liquor is evaporated to obtain a crystallized sodium carbonate product. Antifoam agents are often added in this step to prevent foaming that would foul condensing liquids. These liquids are reused as pure water when clean enough.
The pregnant mother liquor separated from the monohydrate crystals is recycled back to the evaporation units to recover the alkali value therein. Eventually the impurities in the liquors concentrate and a portion must be purged from the evaporation step to meet product quality requirements. The sodium decahydrate crystallization process is one process used to recover the alkali values in the waste purge stream.
Other waste streams and sodium carbonate-containing streams can be cooled using the sodium carbonate decahydrate process to recover alkali values from weak liquor streams. The crystals formed are separated from the weak bittern mother liquor and can be melted and conveniently reintroduced into the monohydrate process or used as feed stock to other sodium crystallization processes such as sodium bicarbonate and sodium sesquicarbonate. The resulting weak bittern mother liquor is also valuable as an alkalinity source for such processes as flue-gas desulfurization. The sodium carbonate decahydrate process is a valuable process for recovering alkali values from sodium carbonate processes.
When the sodium carbonate product is to be used to make sodium bicarbonate, the anhydrous soda ash is dissolved in water and the resultant sodium carbonate solution is then reacted with carbon dioxide to form sodium bicarbonate in accordance with the following reaction:
Na2CO3+CO2+H2Oxe2x86x922NaHCO3 
However, even if treated with activated carbon, objectionably some organic materials from the anhydrous soda ash are passed on to the sodium bicarbonate process. This residual organic material interferes with its ability to react with carbon dioxide.
Thus considerable engineering skill is needed to maximize the carbon dioxide adsorption efficiency of sodium carbonate and the rate of sodium bicarbonate crystal formation from sodium carbonate. An improved method for modifying the sodium carbonate source that enhances the carbonation reaction and avoids animal derivatives would be highly advantageous.
Sodium carbonate produced from the conventionally mined trona ore via the xe2x80x9cmonohydratexe2x80x9d process is known to contain dissolved organic matter and other insoluble materials. The liquor produced by dissolving the crude soda ash is sometimes treated with carbon to remove the dissolved organic matter which may cause foaming, crystal modification, and/or color problems in the final product. Sodium carbonate monohydrate crystals formed in the evaporation process are separated from the mother liquor and sent to the dryers to produce soda ash. The soluble impurities are recycled with the centrate to the crystallizer where they are further concentrated. To maintain final product quality, it eventually becomes necessary to remove the impurities with a crystallizer purge stream.
The purge stream from the evaporation process is sometimes cooled crystallizing sodium carbonate decahydrate and separating the decahydrate crystals to recover the alkali values therein. The decahydrate crystals can be melted and returned to the centrate system, or melted and fed directly to an evaporation unit, or used as a sodium source for the production of saleable sodium salts (e.g. dense soda ash, light soda ash, sodium bicarbonate, or sodium sesquicarbonate.)
The liquors from the separation, purification, and/or purge steps maybe sent to surface evaporation ponds or to abandoned underground mine workings. The sodium carbonate containing liquors from such disposals and/or natural mine waters, can be cooled using the xe2x80x9cdecahydratexe2x80x9d process to improve the purity of the crystals produced while recovering the sodium values therein. Sodium carbonate decahydrate formed when such waste streams are naturally or mechanically cooled can also be melted, filtered and purified, and re-cooled using the xe2x80x9cdecahydratexe2x80x9d process. The recovered alkali value can then be further processed to valuable sodium carbonate salts (e.g. sodium sesquicarbonate, sodium carbonate, or sodium bicarbonate).
The production of sodium carbonate using a combination of monohydrate and decahydrate processes is well known. Purification methods using carbon filtration and chemical additives such as DADMAC, quaternary amines, bentonite clays, and guar gums have been documented and patented.
For example, a method for removing anionic polymers and acidic impurities from aqueous trona solutions prior to crystallization whereby improved crystal formation is achieved is proposed in U.S. Pat. No.4,472,280, to Keeney.
U.S. Pat. No. 3,981,686, to Lobunez teaches a method for clarifying a carbonate process solution containing suspended insolubles so the suspended insolubles will readily settle out of the carbonate process solution.
U.S. Pat. No. 6,270,740 to Shepard, et al, issued in May, 2001 teaches a process comprising adding an amine additive at a rate of at least 0.017 gallons per ton of sodium carbonate produced, prior to the filtration step of the monohydrate process that results in modified sodium carbonate crystals which, when dissolved in water, have increased reactivity with carbon dioxide in the manufacture of sodium bicarbonate.
Presently, these processes include the presence of at least one nitrogen containing cationic compound chosen from the group consisting of water-soluble cationic polymers and/or fatty substituted quaternary ammonium salts.
Certain dietary requirements limit the use of animal derivatives. For instance, Kosher diets restrict among other things, fats derived from swine and other forbidden animals. Compounds produced from bovine bi-products present concern with bovine spongiform encephalopathy (mad cow disease) and other animal transmitted diseases. A process for producing saleable sodium salts without the use of animal derivatives would be beneficial.
The present invention differs from the system of U.S. Pat. No. 6,270,740 to Shepard, et al, in that the system of that patent employs tallow-based or fatty substituted quaternary amines, while the system of the present invention requires a quaternary amine that is free of animal derivatives. In accordance with the invention the two amine groups comprising suitable non-tallow or non-fatty substituted carbon structure, that yield the desired benefits are:
a) Dialkylethoxilated quaternary salts
b) Benzylalkyl quaternary salts.
We have found that in order to achieve the desired result, the non-tallow based, non-fatty substituted amine salt must be employed at a rate of about 0.020 to about 0.040 mols/min. Although these addition rates may be regarded as comparable to the addition rates of the tallow-based amines, the chemistry of the tallow-based amines is found to react substantially differently such that if one adds more than about 0.020 mols/min of the tallow-based amine, there is a marked reduction in the product""s CO2 uptake, as shown by the table below:
It is apparent that, the product reactivity with respect to CO2 decreases once the addition rate of the tallow-based amine goes above 0.021 mols/min.
We have found that the carbonation reaction to form sodium bicarbonate is enhanced when the sodium carbonate is produced using the process of the invention. In accordance with the present process, the addition of particular amounts of a cationic compound, e.g., a quaternary amine, to treat the 25-30% by weight sodium carbonate liquor prior to filtration, results in a modified sodium carbonate liquor product that, when crystallized and converted to any anhydrous product, is more readily and more thoroughly carbonated with CO2 in the production of sodium bicarbonate. The cationic additive reacts with organic materials in the sodium carbonate liquor to form solid polymeric by-products. It is necessary that the treatment with a cationic compound is made prior to filtering the liquor. After filtering to remove the polymeric by-products and other solid materials, the liquor is evaporated or crystallized to produce a purified and modified sodium carbonate. During evaporation, additional antifoam agent may be added to control foaming and to insure the ability to re-use the generated condensate.
In addition to exceeding the maximum CO2 reactivity that is achieved with a tallow-based amine (0.0220 vs. 0.0117 mols/min), the benefits of the present invention include treatment of supplementary recycle streams such as purge, mother liquor, and other waste streams such as mine water, underflow tails, bicarbonate waste, etc. This is beneficial because soda ash produced using these sodium carbonate sources, without the advantages of the invention, will produce crystals with substantially lower reactivity, i.e. with reactivities similar to crystals made using conventional monohydrate, decahydrate, bicarbonate, or sesquicarbonate processes. Use of the present invention will produce crystals with reactivities averaging 0.0220 mols/min CO2 uptake. The benefit of the present invention resides in the manufacture of sodium bicarbonate whereby the reactivity of the manufactured sodium carbonate to CO2 in the conversion of the sodium bicarbonate is increased and without introducing chemicals from animal derivatives.